Internal combustion engine

ABSTRACT

An internal combustion engine includes an intake port configured to generate a swirl in a cylinder, an exhaust port, and a piston. The piston includes a top surface provided in an upper portion of the piston, a cavity provided from the top surface toward a lower portion of the piston around a central axis of the piston, and a connection surface connecting an inner edge of the top surface and an upper end of a side surface of the cavity to each other. The connection surface is provided to be closer to a lower portion side of the piston than the top surface. An area of the connection surface projected on a plane parallel to the top surface is larger on an intake port side than on an exhaust port side.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-055793 filed onMar. 22, 2017 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to an internal combustion engine.

2. Description of Related Art

When a swirl is generated in a cylinder, mixing between a fuel and aircan be promoted, and thus the combustion state of an air-fuel mixture isimproved. As a result, fuel economy can be improved. Known in thisregard is a technique for adjusting the intensity of the swirl byadjusting the shape of an intake port such as a helical port and atangential port in an internal combustion engine provided with a pistonin which a cavity is formed (refer to, for example, Japanese UnexaminedPatent Application Publication No. 2010-270737 (JP 2010-270737 A)).

SUMMARY

It has been found that a smoke generation amount varies, even at thesame swirl intensity, by inclination of the central axis of the swirlwith respect to the central axis of the cylinder. It has also been foundthat the inclination of the central axis of the swirl varies with theintensity of the tumble component and the intensity of the inversetumble component of intake air flowing into the cylinder and is affectedby the shapes of a cylinder head and the cavity as well as the shape ofthe intake port. It has been found that the combustion state is improvedby the inclination of the central axis of the swirl being reduced to themaximum extent possible and the smoke generation amount decreases as aresult of the improvement of the combustion state.

The present disclosure provides an internal combustion engine in whichinclination of the central axis of a swirl with respect to the centralaxis of a cylinder is reduced.

An aspect of the disclosure relates to an internal combustion engineincluding an intake port configured to generate a swirl in a cylinder,an exhaust port, and a piston. The piston includes a top surfaceprovided in an upper portion of the piston, a cavity provided from thetop surface toward a lower portion of the piston around a central axisof the piston, and a connection surface connecting an inner edge of thetop surface and an upper end of a side surface of the cavity to eachother, and the connection surface is provided to be closer to a lowerportion side of the piston than the top surface. An area of theconnection surface projected on a plane parallel to the top surface islarger on an intake port side than on an exhaust port side.

The upper portion of the piston is a part of the piston that is on acylinder head side. The lower portion of the piston is a part of thepiston that is on a crankshaft side. The inner edge of the top surfaceis a boundary between the top surface and the connection surface. Anupper end of a wall surface of the cavity is a boundary between the wallsurface of the cavity and the connection surface. The top surface is aflat surface provided in the upper portion of the piston (In the presentspecification, “flat” includes the meaning of “substantially flat”). Thecavity is a space recessed from the top surface toward the lower portionof the piston. For example, the cavity is a space to which a fuel isinjected. The connection surface is disposed between the inner edge ofthe top surface and the upper end of the side surface of the cavity. Theconnection surface is a surface connected to the inner edge of the topsurface and is a surface provided to be closer to the lower portion sideof the piston than the top surface. The connection surface may be acurved surface or a flat surface inclined with respect to the topsurface.

A flow of intake air in the cylinder may contain a tumble component andan inverse tumble component. The tumble component and the inverse tumblecomponent rotate in opposite directions in the cylinder. The tumblecomponent flows from the intake port toward the exhaust port side mainlythrough the upper portion of the cylinder (that is, the cylinder headside in the cylinder). The inverse tumble component flows through thecylinder from the intake port and along the wall surface of the cylindermainly at an almost vertical angle. Accordingly, the inverse tumblecomponent flows toward the piston at an angle close to a right anglewith respect to the top surface of the piston. When the connectionsurface is projected on a plane parallel to the top surface, the area ofthe top surface decreases as the projected area of the connectionsurface on the intake port side increases, and thus the amount of theinverse tumble component colliding with the top surface decreases andthe amount of the inverse tumble component colliding with the connectionsurface increases. In a case where the inverse tumble component collideswith the top surface of the piston, the intensity of the inverse tumblecomponent is reduced, the inverse tumble component is dispersed, or theinverse tumble component is stagnant on the top surface because theinverse tumble component collides with the piston at an angle close to aright angle. In a case where the inverse tumble component collides withthe connection surface, the inverse tumble component is likely to flowalong the connection surface because the inverse tumble componentcollides with the piston at an angle smaller than a right angle.Accordingly, a reduction in the intensity of the inverse tumblecomponent is suppressed in a case where the projected area of theconnection surface on the intake port side is larger. Then, a largeramount of the inverse tumble component collides with the tumblecomponent, and thus the intensity of the tumble component can be reducedby the inverse tumble component. Therefore, inclination of the centralaxis of the swirl attributable to the tumble component acting on theswirl can be suppressed. In this manner, inclination of the central axisof the swirl with respect to the central axis of the cylinder can bereduced.

In the internal combustion engine according to the aspect of thedisclosure, the intake port may include a tangential port and a helicalport and the area of the connection surface on the intake port sideprojected on the plane parallel to the top surface may be larger on atangential port side than on a helical port side.

In a case where the tangential port and the helical port are formed, theintensity of the tumble component of intake air flowing in from thetangential port is larger than the intensity of the tumble component ofintake air flowing in from the helical port, and thus the intake airflowing in from the tangential port affects the inclination of thecentral axis of the swirl to a larger extent. Accordingly, a largeramount of the inverse tumble component from the tangential port iscapable of colliding with the tumble component by the projected area ofthe connection surface on the tangential port side being increased, andthus the intensity of the tumble component can be further reduced. As aresult, inclination of the central axis of the swirl with respect to thecentral axis of the cylinder can be reduced.

In the internal combustion engine according to the aspect of thedisclosure, the intake port may include a tangential port, and a centralaxis of the inner edge of the top surface may be misaligned to atangential port side from the central axis of the piston.

In this case, the projected area of the connection surface on thetangential port side increases, and thus a reduction in the intensity ofthe inverse tumble component can be suppressed. As a result, theintensity of the tumble component becomes likely to be reduced by theinverse tumble component. In other words, inclination of the centralaxis of the swirl with respect to the central axis of the cylinder canbe reduced. The shape of the inner edge of the top surface may not be aperfect circle. The central axis of the inner edge of the top surface inthis case may be replaced with the center of gravity of the inner edgeof the top surface.

In the internal combustion engine according to the aspect of thedisclosure, the length of a horizontal direction component of theconnection surface may be longer on the intake port side than on theexhaust port side.

In the internal combustion engine according to the aspect of thedisclosure, the height of the side surface of the cavity may be the sameover an entire circumference of the cavity.

According to the aspect of the disclosure, the inclination of thecentral axis of the swirl with respect to the central axis of thecylinder can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments will be described below with reference to theaccompanying drawings, in which like numerals denote like elements, andwherein:

FIG. 1 is a diagram illustrating a schematic configuration of aninternal combustion engine according to an example;

FIG. 2 is a top view of a piston according to a first example;

FIG. 3 is a cross-sectional view of the piston taken along cutting lineof FIG. 2;

FIG. 4 is a diagram showing a time when an upper portion of the pistonaccording to FIGS. 2 and 3 is projected on a plane parallel to a topsurface;

FIG. 5 is a top view of the piston in a case where a central axis of aconnection surface is on a central axis of the piston;

FIG. 6 is a cross-sectional view of the piston taken along cutting lineVI-VI of FIG. 5;

FIG. 7 is a diagram showing a time when the upper portion of the pistonaccording to FIGS. 5 and 6 is projected on a plane parallel to the topsurface;

FIG. 8 is a diagram showing a flow of intake air in the pistonillustrated in FIGS. 5 and 6;

FIG. 9 is a diagram showing the flow of the intake air in the pistonillustrated in FIGS. 5 and 6;

FIG. 10 is a diagram showing the flow of the intake air in the pistonillustrated in FIGS. 2 and 3;

FIG. 11 is a diagram showing the flow of the intake air in the pistonillustrated in FIGS. 2 and 3;

FIG. 12 is a top view of a piston in which a valve recess is formed;

FIG. 13 is a diagram showing a time when an upper portion of the pistonaccording to FIG. 12 is projected on a plane parallel to a top surface;

FIG. 14 is a top view of a piston according to a second example;

FIG. 15 is a diagram showing a time when an upper portion of the pistonaccording to FIG. 14 is projected on a plane parallel to a top surface;

FIG. 16 is a diagram showing a time when a projected area of aconnection surface on an intake port side illustrated in FIG. 15 isfurther separated into a projected area on a helical port side and aprojected area on a tangential port side;

FIG. 17 is a top view of a piston in which a valve recess is formed; and

FIG. 18 is a diagram showing a time when an upper portion of the pistonaccording to FIG. 17 is projected on a plane parallel to a top surface.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments will be illustratively described in detailbased on examples with reference to accompanying drawings. Thedimensions, materials, shapes, relative dispositions, and so on of thecomponent parts described in the examples do not limit the scope of thedisclosure unless otherwise noted.

FIRST EXAMPLE

FIG. 1 is a diagram illustrating a schematic configuration of aninternal combustion engine 1 according to a first example. In the firstexample, illustration of some components is omitted so that the internalcombustion engine 1 is shown in a simple way. The internal combustionengine 1 is mounted in, for example, a vehicle.

A cylinder 2 is formed in a cylinder block 11 of the internal combustionengine 1. An intake port 4 and an exhaust port 5 are formed in acylinder head 12 of the internal combustion engine 1. An intake valve 6is provided in the cylinder 2 side end portion of the intake port 4. Anexhaust valve 7 is provided in the cylinder 2 side end portion of theexhaust port 5. Two intake valves 6 and two exhaust valves 7 aredisposed for each cylinder 2. Accordingly, each of the intake port 4 andthe exhaust port 5 branches into two. At least one of the two intakeports 4 is formed to generate a swirl. The intake port 4 formed togenerate the swirl is, for example, a helical port. At least one of thetwo intake ports 4 is formed to generate a tumble and an inverse tumble.The intake port 4 formed to generate the tumble and the inverse tumbleis, for example, a tangential port. The shape of the intake port 4 isnot limited thereto. The intake port 4 may have any shape insofar as theintake port 4 generates a swirl, a tumble, and an inverse tumble in thecylinder 2.

A piston 8 is disposed in the cylinder 2. FIG. 2 is a top view of thepiston 8 according to the first example. FIG. 3 is a cross-sectionalview of the piston 8 taken along cutting line of FIG. 2. A top surface81 of the piston 8 is a surface that is formed in the upper portion ofthe piston 8 (that is, the cylinder head 12 side of the piston 8) and isa surface that is formed to be orthogonal to a central axis A1 of thepiston 8. A cavity 82 recessed from the top surface 81 toward the lowerportion of the piston 8 is formed around the central axis A1 of thepiston 8. The cavity 82 is formed by a surface including a bottomsurface 82A and a side surface 82B. The bottom surface 82A is a surfacethat is formed to be parallel to the top surface 81. The side surface82B is a surface standing upright from the bottom surface 82A and is acylindrical surface about the central axis A1. A connection surface 83is formed between the top surface 81 and the cavity 82. The connectionsurface 83 is a surface inclined with respect to the central axis Al ofthe piston 8. The connection surface 83 is a surface connecting theupper end of the side surface 82B and an inner edge 81A of the topsurface 81 to each other. Although the connection surface 83 is a flatsurface inclined with respect to the central axis A1 of the piston 8 inFIG. 3, the connection surface 83 may also be a curved surface insteadof the flat surface. The shapes of the bottom surface 82A and the sidesurface 82B are not limited to the shapes illustrated in FIG. 3. Forexample, although the side surface 82B is formed to be parallel to thecentral axis A1 of the piston 8 in FIG. 3, the shape of the side surface82B is not limited thereto and the side surface 82B may also be formedsuch that the inner diameter of the side surface 82B gradually decreasesfrom the lower end of the side surface 82B toward the upper end of theside surface 82B. The side surface 82B in this case may be a curvedsurface. In a case where the side surface 82B is a curved surface, theplace in the side surface 82B where the inner diameter of the sidesurface 82B is smallest may be the upper end of the side surface 82B.The bottom surface 82A and the side surface 82B may be gently connectedto each other as well. Although the places where the intake valve 6 andthe exhaust valve 7 are projected on the piston 8 are indicated bytwo-dot chain lines in FIG. 2, the places may also be where the cylinder2 side end portions of the intake port 4 and the exhaust port 5 areprojected.

The central axis A1 of the piston 8 in FIGS. 2 and 3 is the central axisof the cylinder 2 and the central axis of the cavity 82 as well. A2 inFIGS. 2 and 3 is the central axis of the boundary line between theconnection surface 83 and the top surface 81. In the followingdescription, A2 will be referred to as the central axis of theconnection surface 83. In the first example, the boundary line betweenthe connection surface 83 and the top surface 81 has the shape of acircle about the central axis A2 as illustrated in FIG. 2. However, thecircle may not be a perfect circle. For example, the circle may be anellipse. The boundary line may also have any shape other than a circularshape and an elliptical shape. In that case, the center of gravity maybe on A2.

The connection surface 83 according to the first example is formed suchthat the area projected on a plane that is parallel to the top surface81 is larger on the intake port 4 side (which may also be the intakevalve 6 side) than on the exhaust port 5 side (which may also be theexhaust valve 7 side). In the following description, the connectionsurface 83 that is projected on a plane parallel to the top surface 81will also be referred to as a “plane of projection” and the area of theconnection surface 83 described below will also be referred to as a “projected area”. FIG. 4 is a diagram showing a time when the upperportion of the piston 8 according to FIGS. 2 and 3 is projected on aplane parallel to the top surface 81. The area of the part that isindicated by hatching in FIG. 4 corresponds to the projected area, 83Acorresponds to the projected area of the connection surface 83 on theintake port 4 side, and 83B corresponds to the projected area of theconnection surface 83 on the exhaust port 5 side. The intake port 4 siderefers to the side that includes the intake port 4 when the piston 8 isdivided into two with one plane passing through the central axis A1 ofthe piston 8 such that the piston 8 is separated into the side thatincludes the intake valve 6 and the side that includes the exhaust valve7. Likewise, the exhaust port 5 side refers to the side that includesthe exhaust port 5 when the piston 8 is divided into two with one planepassing through the central axis A1 of the piston 8 such that the piston8 is separated into the side that includes the intake valve 6 and theside that includes the exhaust valve 7. In the following description,the connection surface on the intake port 4 side will also be referredto as a range close to the intake port 4 and the connection surface onthe exhaust port 5 side will also be referred to as a range close to theexhaust port 5.

It can be said that the central axis A2 of the connection surface 83according to the first example is misaligned (eccentric) to the intakevalve 6 side with respect to the central axis A1 of the piston 8. Thelength of the horizontal direction component of the connection surfacediffers on the intake port 4 side and the exhaust port 5 side to thesame extent as the central axis A2 of the connection surface 83 ismisaligned to the intake valve 6 side with respect to the central axisA1 of the piston 8. In other words, in FIG. 3, the length of thehorizontal direction component of the connection surface 83 isrelatively short as indicated by Ll on the exhaust valve 7 side and isrelatively long as indicated by L2 on the intake valve 6 side. A heightH1 of the side surface 82B is the same in the entire cavity 82.

FIG. 5 is a top view of the piston 8 in a case where the central axis A2of the connection surface 83 is on the central axis A1 of the piston 8.FIG. 6 is a cross-sectional view of the piston 8 taken along cuttingline VI-VI of FIG. 5. FIG. 7 is a diagram showing a time when the upperportion of the piston 8 according to FIGS. 5 and 6 is projected on aplane parallel to the top surface 81. The area of the part that isindicated by hatching in FIG. 7 corresponds to the projected area, 83Acorresponds to the projected area of the connection surface 83 on theintake port 4 side, and 83B corresponds to the projected area of theconnection surface 83 on the exhaust port 5 side. In this case, theconnection surface 83 has the same projected area on the intake valve 6side and the exhaust valve 7 side. As illustrated in FIG. 6, a length L3of the horizontal direction component of the connection surface 83 onthe exhaust valve 7 side is equal to a length L4 of the horizontaldirection component of the connection surface 83 on the intake valve 6side. The height H1 of the side surface 82B is the same in the entirecavity 82.

In the piston 8 illustrated in FIGS. 2 and 3 and the piston 8illustrated in FIGS. 5 and 6, a difference in inverse tumble componentintensity occurs due to the different shapes of the connection surface83 described above. FIGS. 8 and 9 are diagrams showing the flow ofintake air in the piston 8 illustrated in FIGS. 5 and 6. In FIGS. 8 and9, TA represents a tumble component and TB represents an inverse tumblecomponent. The tumble component TA is a flow from the intake port 4toward the exhaust port 5 side and mainly through the upper portion ofthe cylinder 2, and the inverse tumble component TB is a flow from theintake port 4 and along the wall surface of the cylinder 2 and the maindirection of the inverse tumble component TB is the vertical direction.A swirl is mainly a flow around the central axis of the cylinder 2.Since the main direction of the inverse tumble component TB is thevertical direction, the inverse tumble component TB collides with thetop surface 81 of the piston 8 at an angle close to a right angle.Accordingly, the intensity of the inverse tumble component TB is likelyto be reduced, the inverse tumble component TB is likely to bedispersed, and the inverse tumble component TB is likely to be stagnanton the top surface 81. Then, the tumble component TA becomes relativelystronger and mainly the flow of the tumble component TA remains in thecylinder 2 as illustrated in FIG. 9. The stronger the flow of the tumblecomponent TA, the more inclined the central axis of the swirl withrespect to the central axis A1 of the piston 8. Combustion statedeterioration is likely to result from the inclination of the centralaxis of the swirl.

FIGS. 10 and 11 are diagrams showing the flow of intake air in thepiston 8 illustrated in FIGS. 2 and 3. In a case where a projected area83A of the connection surface 83 on the intake port 4 side is large, alarger amount of the inverse tumble component TB collides with theconnection surface 83. In this case, the inverse tumble component TBcollides with the connection surface 83 at an angle smaller than a rightangle, and thus the inverse tumble component TB is likely to flow alongthe connection surface 83. Accordingly, a reduction in the intensity ofthe inverse tumble component TB is suppressed even after the inversetumble component TB collides with the piston 8. Then, the tumblecomponent TA and the inverse tumble component TB collide with each otherand counteract each other as illustrated in FIG. 11. As a result, theintensity of the tumble component TA decreases, and thus inclination ofthe central axis of the swirl by the tumble component TA can besuppressed and combustion state deterioration can be suppressed.

An overall increase in the projected area of the connection surface 83is conceivable as well. In that case, however, the intensity of thetumble component is not reduced with ease as the intensity of the tumblecomponent also increases. As the area of the top surface 81 as a wholedecreases, a squishing effect decreases. In the first example, incontrast, the projected area of the connection surface 83 on the exhaustport 5 side is relatively small, and thus the area of the top surface 81on the exhaust port 5 side is large. As a result, the intensity of thetumble component can be reduced and a decline in squishing effect can besuppressed.

In some cases, a valve recess is formed in the piston 8. FIG. 12 is atop view of the piston 8 in which a valve recess is formed. FIG. 13 is adiagram showing a time when the upper portion of the piston 8 accordingto FIG. 12 is projected on a plane parallel to the top surface. The areaof the part that is indicated by hatching in FIG. 13 corresponds to theprojected area, 83A corresponds to the projected area of the connectionsurface 83 on the intake port 4 side, and 83B corresponds to theprojected area of the connection surface 83 on the exhaust port 5 side.An intake valve recess 61 corresponding to each intake valve 6 and anexhaust valve recess 71 corresponding to each exhaust valve 7 aredisposed in the piston 8. Even in this case, the connection surface 83is formed such that the projected area 83A of the connection surface 83on the intake port 4 side (which may also be the intake valve 6 side) islarger than the projected area 83B of the connection surface 83 on theexhaust port 5 side (which may also be the exhaust valve 7 side). Evenin a case where the valve recess is formed in the piston 8 as describedabove, a reduction in the intensity of the inverse tumble component,dispersion of the inverse tumble component, and stagnation of theinverse tumble component on the top surface 81 can be suppressed by thearea of the connection surface 83 in the range close to the intake valve6 being relatively increased. As a result, the intensity of the tumblecomponent TA can be reduced by the inverse tumble component TB.

In the first example, an increase in the projected area of theconnection surface 83 results in the same degree of decrease in the areaof the top surface 81. Accordingly, it can be said that the top surface81 and the connection surface 83 are formed such that the area of thetop surface 81 is smaller on the intake port 4 side than on the exhaustport 5 side.

As described above, according to the first example, inclination of thecentral axis of the swirl can be reduced by the projected area of theconnection surface 83 being larger on the intake port 4 side (which mayalso be the intake valve 6 side) than on the exhaust port 5 side (whichmay also be the exhaust valve 7 side). As a result, combustion can becarried out well.

SECOND EXAMPLE

FIG. 14 is a top view of the piston 8 according to a second example. Inthe second example, one of the intake ports 4 is a helical port and theother intake port 4 is a tangential port. The helical port side intakevalve 6 will be referred to as a helical intake valve 6A and thetangential port side intake valve 6 will be referred to as a tangentialintake valve 6B. In the second example, the helical port is optional.

FIG. 15 is a diagram showing a time when the upper portion of the piston8 according to FIG. 14 is projected on a plane parallel to the topsurface 81. The area of the part that is indicated by hatching in FIG.15 corresponds to the projected area, 83A corresponds to the projectedarea of the connection surface 83 on the intake port 4 side, and 83Bcorresponds to the projected area of the connection surface 83 on theexhaust port 5 side. The projected area of the connection surface 83according to the second example is larger on the intake port 4 side(which may also be the intake valve 6 side) than on the exhaust port 5side (which may also be the exhaust valve 7 side). FIG. 16 is a diagramshowing a time when the upper portion of the piston 8 according to FIG.14 is projected on a plane parallel to the top surface 81. FIG. 16 is adiagram showing a time when the projected area 83A of the connectionsurface 83 on the intake port 4 side illustrated in FIG. 15 is furtherseparated into a projected area 83AA on the helical port side and aprojected area 83AB on the tangential port side. In the second example,the projected area 83AB of the connection surface 83 on the tangentialport side (that is, the tangential intake valve 6B side) is larger thanthe projected area 83AA of the connection surface 83 on the helical portside (that is, the helical intake valve 6A side). The helical port siderefers to the side that includes the helical port (the helical intakevalve 6A) when the piston 8 is divided into two with one plane passingthrough the central axis A1 of the piston 8 such that the piston 8 isseparated into the side that includes the intake valve 6 and the sidethat includes the exhaust valve 7 and then the piston 8 is divided intofour by the piston 8 being separated into the side that includes thehelical intake valve 6A and the side that includes the tangential intakevalve 6B with one plane orthogonal to the plane and passing through thecentral axis A1 of the piston 8. Likewise, the tangential port siderefers to the side that includes the tangential port (the tangentialintake valve 6B) when the piston 8 is divided into two with one planepassing through the central axis A1 of the piston 8 such that the piston8 is separated into the side that includes the intake valve 6 and theside that includes the exhaust valve 7 and then the piston 8 is dividedinto four by the piston 8 being separated into the side that includesthe helical intake valve 6A and the side that includes the tangentialintake valve 6B with one plane orthogonal to the plane and passingthrough the central axis A1 of the piston 8. The second example isidentical to the first example when it comes to the interpretation ofthe intake port 4 side and the exhaust port 5 side.

It can be said that the central axis A2 of the connection surface 83according to the second example is misaligned (eccentric) from thecentral axis A1 of the piston 8 to the tangential intake valve 6B side(that is, the tangential port side). Since the central axis A2 of theconnection surface 83 is misaligned with respect to the central axis Alof the piston 8, the length of the horizontal direction component of theconnection surface 83 on the tangential intake valve 6B side isrelatively long. The height of the side surface 82B is the same in theentire cavity 82 as illustrated in FIG. 3. The inner edge 81A of the topsurface 81 may have a shape other than a circular shape. In that case,the center of gravity of the shape may be on A2.

For example, 20% of the tumble component in the cylinder 2 is fromintake air flowing in from the helical port and 80% is from intake airflowing in from the tangential port. Accordingly, a reduction in theintensity of the inverse tumble component from the tangential port needsto be suppressed for a reduction in the intensity of the tumblecomponent. The intensity of the inverse tumble component from thetangential port can be increased by the projected area 83AB of theconnection surface 83 on the tangential intake valve 6B side beingrelatively increased, and thus the intensity of the tumble component canbe effectively decreased.

In some cases, a valve recess is formed in the piston 8. FIG. 17 is atop view of the piston 8 in which a valve recess is formed. A helicalintake valve recess 61A corresponding to the helical intake valve 6A, atangential intake valve recess 61B corresponding to the tangentialintake valve 6B, and the exhaust valve recess 71 corresponding to eachexhaust valve 7 are disposed in the piston 8. Even in this case, theprojected area of the connection surface 83 is larger on the intake port4 side (which may also be the intake valve 6 side) than on the exhaustport 5 side (which may also be the exhaust valve 7 side). FIG. 18 is adiagram showing a time when the projected area of the connection surface83 on the intake port 4 side illustrated in FIG. 17 is separated intothe projected area 83AA on the helical port side and the projected area83AB on the tangential port side. The projected area 83AB of theconnection surface 83 on the tangential port side (that is, thetangential intake valve 6B side) is larger than the projected area 83AAof the connection surface 83 on the helical port side (that is, thehelical intake valve 6A side). Even in a case where the valve recess isformed in the piston 8 as described above, a reduction in the intensityof the inverse tumble component can be suppressed by the projected area83AB of the connection surface 83 on the tangential intake valve 6B sidebeing relatively increased. As a result, the intensity of the tumblecomponent can be reduced by the inverse tumble component.

In the second example, an increase in the projected area of theconnection surface 83 results in the same degree of decrease in the areaof the top surface 81. Accordingly, it can be said that the area of thetop surface 81 on the tangential intake valve 6B side is relativelysmall in the second example.

As described above, according to the second example, inclination of thecentral axis of the swirl can be reduced by the projected area of theconnection surface 83 on the tangential intake valve 6B side beingrelatively increased. As a result, combustion can be carried out well.Since the projected area of the connection surface 83 on the exhaustport 5 side and the projected area of the connection surface 83 on thehelical port side are relatively small, the intensity of the tumblecomponent can be reduced and a decline in squishing effect can besuppressed.

What is claimed is:
 1. An internal combustion engine comprising: anintake port configured to generate a swirl in a cylinder; an exhaustport; and a piston, wherein: the piston includes a top surface providedin an upper portion of the piston, a cavity provided from the topsurface toward a lower portion of the piston around a central axis ofthe piston, and a connection surface connecting an inner edge of the topsurface and an upper end of a side surface of the cavity to each other,and the connection surface being provided to be closer to a lowerportion side of the piston than the top surface; and an area of theconnection surface projected on a plane parallel to the top surface islarger on an intake port side than on an exhaust port side.
 2. Theinternal combustion engine according to claim 1, wherein: the intakeport includes a tangential port and a helical port; and the area of theconnection surface on the intake port side projected on the planeparallel to the top surface is larger on a tangential port side than ona helical port side.
 3. The internal combustion engine according toclaim 1, wherein: the intake port includes a tangential port; and acentral axis of the inner edge of the top surface is misaligned to atangential port side from the central axis of the piston.
 4. Theinternal combustion engine according to claim 1, wherein a length of ahorizontal direction component of the connection surface is longer onthe intake port side than on the exhaust port side.
 5. The internalcombustion engine according to claim 1, wherein a height of the sidesurface of the cavity is the same over an entire circumference of thecavity.